Understanding RF Spectrum Analysis

Connect802 has RF engineers trained and ready to help you with your wireless network project. The application of Wi-Fi, WiMAX, and WRAN RF spectrum analysis, using full-featured RF spectrum analyzers, provides definitive information about signal quality, noise and interference. Our sales team can also discuss the tactical RF spectrum analysis tools that we sell.

If you have questions, please don't hesitate to call us today! We'll be happy to provide you with any technical explanation that you need to help you assure a successful wireless networking deployment.

An RF spectrum analyzer is an instrument that displays signal energy at each frequency step across a transmission band. The most commonly recognized display is the "FFT" (Fast Fourier Transform) showing the spectral signature of a transmission. "Swept Spectrogram" displays, also called "Waterfall Displays" show energy levels over time and frequency together in one representation. An RF engineer evaluates the spectrum analyzer displays to draw conclusions about the quality of the transmission and accompanying noise and interference.

Spectrum analysis may be compared and contrasted with site survey measurements made using utilities and tools based solely on 802.11 measurements.

A spectrum analyzer is a specialized piece of equipment that measures RF signal energy across a range of frequencies. Spectrum analysis can be used in harmony with 802.11 channel analysis to provide a complete picture of the radio transmission and reception characteristics of a particular environment.

To best understand how spectrum analysis measurements differ from 802.11 channel measurements it’s beneficial to first understand exactly what’s meant by “802.11 channel measurement.” Site survey and monitoring software that presents information obtained from an 802.11 chipset can (..and this should be self-evident.). only manipulate information obtained from the 802.11 chipset. Hence, tools like AirMagnet, AiroPeek and NetStumbler, along with vendor utilities that present signal strength and/or Signal-to-Noise ratios like the Cisco AiroNet utility (and other similar utilities) are limited to the 802.11 channel space. That is, they are only capable of measuring RF conditions that present themselves in such a way that the 802.11 radio receiver and chipset can discern them. These utilities can report the presence of other 802.11 devices and can measure the strength of a received 802.11 signal, but they can NOT measure background RF energy in the environment.

The fact that 802.11-based tools are incapable of directly measuring background RF energy means that they are incapable of reporting the existence of other (non-802.11) devices that may be operating in the same band as the 802.11 network. This other background energy may come from cordless phones, microwave ovens, and (perhaps most significantly) from the channel “bleed over” that is present in all 802.11 networks.

In the 802.11 realm one often talks about “Channel 1, 6 and 11” as being “non-overlapping.” This is not a completely accurate statement. In fact, the specifications for an 802.11 network define only a center frequency for each channel assignment and then specify the bandwidth (above and below that center frequency) where the signal must drop by 3 dB, and then again by another 3 dB, and so forth. What’s defined is technically known as a “spectral mask” and the implication is that any 802.11 transmitter can generate energy in any part of the 802.11 band. The real-world effect is that when a sufficient number of 802.11 transmitters are present in an environment the transmission band develops a baseline of background energy that can spread across the entire band. The result of this background energy is that an 802.11 receiver can become “confused” as to what is the intended signal and what is the unintended background signal. Technically this is referred to as a condition where “coherent background signals” cause a receiver to “lose lock”. The result is that data is lost, retransmissions cause effective data rates to drop, and users can lose connections.

802.11-based tools are incapable of detecting background energy and the aggregate effect of multiple, overlapping spectral masks. Only a spectrum analyzer can do this.

How Is Spectrum Analysis Performed?

The distribution of energy across the frequency range assigned to a transmission channel is defined in the appropriate IEEE specification (i.e. 802.11b, 802.11g, 802.11a, 802.11n). One type of specification is a "spectral mask" (shown to the right). Other specifications relate to various aspects of the signals characteristics.

An RF engineer uses a spectrum analyzer to capture a representation of the signal energy in graphic format, as shown in the two images below. The engineer has the training and experience to be able to identify the characteristics of the spectral display and draw conclusions related to the quality of the signal, signal strength, reflections, noise and interference.

The 802.11b 2.4 GHz Spectral Mask

802.11b data is seen being transmitted on a single channel using BPSK (Binary Phase Shift Keying).

802.11a data is seen being transmitted in the 5 GHz frequency band using OFDM (Orthogonal Frequency Division Multiplexing). This is the same modulation technique used by 802.11g in the 2.4 GHz band.